Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors has continued to advance at a great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these devices.
For high-speed image sensors, a global shutter can be used to capture fast-moving objects. A global shutter typically enables all pixel cells in the image sensor to simultaneously capture the image. For slower moving objects, the more common rolling shutter is used. A rolling shutter normally captures the image in a sequence. For example, each row within a two-dimensional (“2D”) pixel cell array may be enabled sequentially, such that each pixel cell within a single row captures the image at the same time, but each row is enabled in a rolling sequence. Accordingly, each row of pixel cells captures the image during a different image acquisition window. For slow moving objects the time differential between each row generates image distortion. For fast-moving objects, a rolling shutter causes a perceptible elongation distortion along the object's axis of movement.
To implement a global shutter, storage structures can be used to temporarily store the image charge acquired by each pixel cell in the array while it awaits readout from the pixel cell array. Factors that affect performance in an image sensor pixel cell having a global shutter include shutter efficiency, dark current, white pixels and image lag. Moreover pollution of image charge in the storage structure may have a deleterious effect on image quality.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.